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From 53c0d0dcad838ef8b9b0faf0c8066d47380f2cf1 Mon Sep 17 00:00:00 2001
From: Alex Williamson <>
Date: Thu, 13 Dec 2018 21:55:04 +0100
Subject: [PATCH 3/5] kvm: check KVM_CAP_SYNC_MMU with kvm_vm_check_extension()

RH-Author: Alex Williamson <>
Message-id: <154473810399.22725.17809569801460658619.stgit@gimli.home>
Patchwork-id: 83495
O-Subject: [RHEL-7.7 qemu-kvm PATCH 3/5] kvm: check KVM_CAP_SYNC_MMU with kvm_vm_check_extension()
Bugzilla: 1659229
RH-Acked-by: Peter Xu <>
RH-Acked-by: Cornelia Huck <>
RH-Acked-by: Auger Eric <>

From: Greg Kurz <>

Bugzilla: 1659229
Notes: We don't have kvm_vm_check_extension() but we can still
       cache the result, which gives us the same hook to trigger
       the balloon inhibitor here.

On a server-class ppc host, this capability depends on the KVM type,
ie, HV or PR. If both KVM are present in the kernel, we will always
get the HV specific value, even if we explicitely requested PR on
the command line.

This can have an impact if we're using hugepages or a balloon device.

Since we've already created the VM at the time any user calls
kvm_has_sync_mmu(), switching to kvm_vm_check_extension() is
enough to fix any potential issue.

It is okay for the other archs that also implement KVM_CAP_SYNC_MMU,
ie, mips, s390, x86 and arm, because they don't depend on the VM being
created or not.

While here, let's cache the state of this extension in a bool variable,
since it has several users in the code, as suggested by Thomas Huth.

Signed-off-by: Greg Kurz <>
Message-Id: <150600965332.30533.14702405809647835716.stgit@bahia.lan>
Reviewed-by: David Gibson <>
Signed-off-by: Paolo Bonzini <>
(cherry picked from commit 62dd4edaaf859b60f74a51f2a526d4d3d85d0248)
Signed-off-by: Miroslav Rezanina <>
 include/sysemu/kvm.h | 2 +-
 kvm-all.c            | 7 +++++--
 kvm-stub.c           | 4 ++--
 3 files changed, 8 insertions(+), 5 deletions(-)

diff --git a/include/sysemu/kvm.h b/include/sysemu/kvm.h
index 49cfc42..e4403be 100644
--- a/include/sysemu/kvm.h
+++ b/include/sysemu/kvm.h
@@ -135,7 +135,7 @@ extern KVMState *kvm_state;
 int kvm_init(void);
-int kvm_has_sync_mmu(void);
+bool kvm_has_sync_mmu(void);
 int kvm_has_vcpu_events(void);
 int kvm_has_robust_singlestep(void);
 int kvm_has_debugregs(void);
diff --git a/kvm-all.c b/kvm-all.c
index 9486b9a..f5b7958 100644
--- a/kvm-all.c
+++ b/kvm-all.c
@@ -92,6 +92,7 @@ struct KVMState
     int xsave, xcrs;
     int many_ioeventfds;
     int intx_set_mask;
+    bool sync_mmu;
     /* The man page (and posix) say ioctl numbers are signed int, but
      * they're not.  Linux, glibc and *BSD all treat ioctl numbers as
      * unsigned, and treating them as signed here can break things */
@@ -1479,6 +1480,8 @@ int kvm_init(void)
     cpu_interrupt_handler = kvm_handle_interrupt;
+    s->sync_mmu = !!kvm_check_extension(kvm_state, KVM_CAP_SYNC_MMU);
     return 0;
@@ -1775,9 +1778,9 @@ int kvm_vcpu_ioctl(CPUState *cpu, int type, ...)
     return ret;
-int kvm_has_sync_mmu(void)
+bool kvm_has_sync_mmu(void)
-    return kvm_check_extension(kvm_state, KVM_CAP_SYNC_MMU);
+    return kvm_state->sync_mmu;
 int kvm_has_vcpu_events(void)
diff --git a/kvm-stub.c b/kvm-stub.c
index 22eaff0..ca6ddd7 100644
--- a/kvm-stub.c
+++ b/kvm-stub.c
@@ -59,9 +59,9 @@ int kvm_cpu_exec(CPUArchState *env)
     abort ();
-int kvm_has_sync_mmu(void)
+bool kvm_has_sync_mmu(void)
-    return 0;
+    return false;
 int kvm_has_many_ioeventfds(void)